At Bombardier in Tucson they have aircraft-specific leading edge covers that extend up the winglets and attach with suction cups. I'm not sure what they are made of, but they seem to be pretty heavy duty and likely to protect the leading edge from most conceivable damage.
Perhaps it's not as big a concern around aircraft with high wings, but I can certainly imagine a tool cart rolling into a Learjet wing. As they say, "An ounce of prevention..."